Single lung transplantation remains limited by a severe shortage of suitable donor lungs. Potential lung donors are often deemed unsuitable because accepted criteria (both lungs clear on the chest roentgenogram, arterial oxygen tension greater than 300 mm Hg with an inspired oxygen fraction of 1.0, a positive end-expiratory pressure of 5 cm H2O, and no purulent secretions) do not distinguish between unilateral and bilateral pulmonary disease. Many adequate single lung grafts may be discarded as a result of contralateral aspiration or pulmonary trauma. We have recently used intraoperative unilateral ventilation and perfusion to assess single lung function in potential donors with contralateral lung disease. In the 11-month period ending October 1, 1990, we performed 18 single lung transplants. In four of these cases (22%), the donor chest roentgenogram or bronchoscopic examination demonstrated significant unilateral lung injury. Donor arterial oxygen tension, (inspired oxygen fraction 1.0; positive end-expiratory pressure 5 cm H2O) was below the accepted level in each case (246 ± 47 mm Hg, mean ± standard deviation). Through the sternotomy used for multiple organ harvest, the pulmonary artery to the injured lung was clamped. A double-lumen endotracheal tube or endobronchial balloon occlusion catheter was used to permit ventilation of the uninjured lung alone. A second measurement of arterial oxygen tension (inspired oxygen fraction 1.0; positive end-expiratory pressure 5 cm H2O) revealed excellent unilateral lung function in all four cases (499.5 ± 43 mm Hg; p < 0.0004). These single lung grafts (three right, one left) were transplanted uneventfully into four recipients (three with pulmonary fibrosis and one with primary pulmonary hypertension). Lung function early after transplantation was adequate in all patients. Two patients were extubated within 24 hours. There were two late deaths, one caused by rejection and Aspergillus infection and the other caused by cytomegalovirus 6 months after transplantation. Two patients are alive and doing well. We conclude that assessment of unilateral lung function in potential lung donors is indicated in selected cases, may be quickly and easily performed, and may significantly increase the availability of single lung grafts.